1. Field of the Invention
The present invention relates to a sheet folding for folding a sheet or recording medium or a sheet stack carrying images thereon, a sheet finisher constructed integrally with or operatively connected to an image forming apparatus for sorting, stacking, stapling, center-stapling, folding or otherwise finishing the sheet or the sheet stack, and an image forming system consisting of the sheet finisher and image forming apparatus.
2. Description of the Background Art
A sheet finisher positioned at the downstream side of a copier, printer or similar image forming apparatus for stapling or otherwise finishing a sheet stack is well known in the art.
It is a common practice with a sheet finisher to staple a sheet stack at the center and then fold the sheet stack in two at the center. To fold the sheet stack, use is made of a fold plate configured to push the stapled portion of the sheet stack toward the nip of a pair of fold rollers while contacting the sheet stack substantially perpendicularly thereto. The sheet stack thus pushed is passed through the nip between the fold rollers and folded thereby. The prerequisite with such center stapling and center folding is that the sheet stack be folded by the fold roller pair at the position accurately coincident with the stapled position. A sheet stack so folded is attractive and can be smoothly spread.
To meet the above requisite, it has been proposed to press the two fold rollers with respective springs in such a manner as to maintain the nip between the rollers at the center of a folding position. Japanese Patent Laid-Open Publication Nos. 2000-143088 and 2000-211805, for example, each use a pair of fold rollers for folding the center of a sheet stack in the direction of conveyance and sheet pushing means including a push plate that pushes the fold of the sheet stack toward the nip between the fold rollers while moving toward the nip. The fold rollers are constantly biased toward each other by biasing means.
Before starting pressing the sheet stack, the fold rollers are held stationary at preselected positions by a stop member, which is fixed in place, while adjoining or lightly contacting each other so as not to exert excessive pressure. In this condition, the push plate is accurately moved toward the nip between the fold rollers to thereby accurately fold the sheet stack. Further, after the trailing edge of the sheet stack has moved away from the fold rollers, the fold rollers are prevented from hitting against each other.
However, in the configuration described above, the fold rollers adjoin or lightly contact each other only when a sheet stack is absent therebetween. When a sheet stack enters the nip between the fold rollers, the weight of the lower fold roller acts in a direction in which the force of the associated spring decreases while the weight of the upper fold roller acts in a direction in which the force of the associated spring increases. As a result, the pressing forces of the springs are brought out of balance with each other. Consequently, the lower fold roller is spaced from the pressing position more than the upper fold roller, so that the nip between the fold rollers is shifted from the center of the pressing position. This prevents the folded position of a sheet stack from accurately coinciding with the stapled position of the same and thereby makes the folded sheet stack unattractive.
Japanese Patent Laid-Open Publication No. 10-279177 discloses a sheet folding device configured to move, when a sheet stack is passed through a pair of fold rollers, one fold roller in unison with, but in the opposite direction to, the other fold roller. This, according to the above document, allows the center of a gap formed between the fold rollers to constantly coincide with the position of a line tangential to both of the rollers when the rollers contact each other. More specifically, a pair of swing arms respectively support the fold rollers at one end thereof and are swingable about respective fulcrums at the other end. A gear train or similar connecting means is arranged between the fulcrums to cause, when one of the swing arms swing, the other swing arm to swing in the opposite direction by the same angle. In this configuration, the fold rollers press a sheet stack while moving symmetrically to each other with respect to the center of a fold, so that the sheet stack can be folded at the same position as the stapled position.
However, a problem with the above sheet folding device is that the gear train or similar connecting means cannot be accurately positioned due to backlash and irregularity among parts. This not only makes it difficult to allow the folded position to accurately coincide with the stapled position, but also makes the resulting sheet stack unattractive.
Further, to sharply fold a sheet stack, it is generally necessary to increase the pressing force in accordance with the thickness of the sheet stack. However, the gear train or similar connecting means, used to move the fold rollers, is limited in strength, so that the pressing force of the fold roller pair must also be relatively weak. Consequently, the range over which the fold rollers can fold a sheet stack is limited. A sheet stack with a loose fold is not only unattractive as a bound matter, but also collapses when stacked together with the other sheet stacks.
Moreover, a fold roller pair, exerting a strong pressing force, is optimum in sharpening the fold of a sheet stack. However, when a sheet stack folded by such a fold roller pair is conveyed, the fold of the sheet stack again expands outward and again becomes loose because the innermost sheet is conveyed more than the outermost sheet little by little. In the worst case, the sheet stack is creased or broken. In addition, the fold rollers are apt to hit against each other when the trailing edge of the sheet stack moves away from the nip. The resulting impact is likely to damage the finisher while producing noise.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2000-211805.